Quantitative assessment of solid waste treatment systems in the industrial ecology perspective by exergy analysis

Solid waste treatment options (recycling, incineration, and landfilling; the two latter processes both with co-generation of heat and electricity) have been studied for cardboard, newspaper, polyethylene, poly(ethylene terephthalate), polypropylene, polystyrene, and poly(vinyl chloride) waste. The conversion processes have been analyzed in terms of the second law of thermodynamics. The analysis allows calculating the exergy (useful energy) embodied in conversion products that can be obtained from the required inputs for the treatment processes. Taking into account the waste materials and the resources to convert them, it proved that recycling is the most efficient option for polyethylene with an efficiency of 62.5% versus 43.6% for incineration and 0.9% for landfilling. Next, waste treatment has been put into the broader perspective of industrial ecology. Exergetic efficiencies of industrial metabolic options have been calculated. Here resources for manufacturing and converting solid products have been considered. Furthermore, selection of one type of conversion excludes the generation of other potential conversion products. Therefore, it has to be taken into account that these latter products still have to be produced starting from virgin resources. Recycling proved to be the most efficient strategy: the ratio eta between exergy embodied in all delivered products on one hand, and all exergy withdrawn from the ecosphere or from waste materials on the other hand, is the highest. For polyethylene, eta proved to be 0.568, whereas eta is 0.503 and 0.329 for incineration and landfilling, respectively. On the other hand, if R the ratio between exergy of delivered products on one hand and exergy of virgin materials on the other hand is calculated, the differences between the industrial metabolic options are larger. Recycling polyethylene showed a ratio R of 0.936, whereas ratios of 0.772 and 0.531 were found for incineration and landfilling, respectively. It has been shown that the exergy concept allows a quantitative comparison of different industrial metabolic options, contributing to a better assessment of sustainability of technology with respect to resource management.